Aromatic polyamide and polyimide resins have found wide applicability as shaped articles in applications where unusual strength under high temperature conditions is needed. However, these polymeric materials are non-melt fabricable, since they tend to degrade far below their crystalline melting points. A variety of techniques has therefore been developed to form these polymers into useful shaped articles.
One such forming technique useful for preparing thin polyimide articles such as films involves casting a polymeric intermediate into the desired final configuration before completing the chemical reaction to the aromatic polyimide. Larger molded shapes can be prepared by molding powders of these polymers at elevated temperatures and pressures.
Continuous forming of such polymeric materials has recently been made possible through the invention described in copending application Ser. No. 971,675 filed Dec. 21, 1978 now U.S. Pat. No. 4,238,538 issued Dec. 9, 1980. The invention described therein lies in the use of a die profile which results in compaction, back pressure and pressure relief, consecutively, in a non-melt fabricable polymeric resin forced through the die by a ram. This die design permits the extrusion of a shaped article of a non-melt fabricable polymeric resin which can then be sintered to form a finished article having unusual strength under high temperature conditions.
In the forming apparatus described above, optimum performance is attained by the inclusion of lubricating fillers such as particulate polytetrafluoroethylene in the resin blend. The inclusion of these lubricating components in the resin blend decreases the ram force required in the apparatus. Inclusion of these additives, in continuously formed articles, however, creates other difficulties. The presence of the internal lubricant weakens the final molded product. In addition, the propensity of polytetrafluoroethylene to agglomerate can result in inhomogeneity in the finished products. In extreme cases, agglomerated particles of polytetrafluoroethylene pull out during subsequent machining of a shaped article, leaving voids.
Accordingly, a continuing need exists for an apparatus for forming polymers such as aromatic polyamide and polyimide resins while reducing the need for lubricating additives in the resin blend.